Many examples of Motor Operated Valve (MOV) stem thrust measurement systems are found in the prior art which utilize strain gage based transducers in various configurations to measure stem thrust during test conditions. During testing in one such system, secondary indicators (such as spring pack displacement) are correlated to stem thrust measurements taken with a strain gage type load cell. Then, under actual operating conditions, the secondary indicator is used to measure operating stem thrust, based on the test measurement correlations.
A prime example of this type system is the Charbonneau et al Analysis and Testing System disclosed in U.S. Pat. No. 4,542,649, incorporated herein by this reference (Charbonneau '649), which uses a compressive load cell to measure stem thrust as the valve stem moves in the valve opening direction. Spring pack displacement is measured simultaneously with the stem thrust measurement, allowing the relationship between the two values to be measured. This correlation is used subsequently to derive stem thrust values from spring pack displacement measurements. Another prior art device associated with MOVs, Crass (U.S. Pat. No. 4,570,903), though more limited in its analysis and testing than Charbonneau, uses a tension load cell to measure stem thrust as the valve stem moves in the valve closing direction for calibration with a secondary indicator.
Industry preference has leaned toward placement of sensors as near (in the mechanical chain) as possible to the actual stem thrust and for measurement throughout the valve cycle. Leon (U.S. Pat. No. 4,805,451) has attached a strain sensing device to the exterior of the valve yoke in an attempt to measure axial stresses in the valve yoke. Branam et al (U.S. Pat. No. 4,856,327) utilizes load cells, clamped under a compressive preload between the actuator and valve yoke, in an attempt to measure tensile and compressive reaction forces which are described in that patent as being proportional to axial stem thrust.
While these systems provide a potential method of continual monitoring of stem thrust, at close proximity to the valve stem, they experience certain drawbacks. For example, without limitation, they are not installed as a full load bearing member between the actuator and valve, therefore, they must be individually calibrated for each installation to determine how readings relate to actual stem load.
Leon uses a strain sensing device attached to the exterior of the valve yoke, mounted to one of the two yoke arms. Stem thrust reaction is carried by the two yoke arms, but is not necessarily distributed equally throughout that structure. Strain measurements from the valve yoke are used to infer stem thrust; compression in the yoke is a reaction to tension in the valve stem, and vice versa. However, the Leon sensing device must be attached to a valve yoke and calibrated for that particular yoke, by applying a known load to the yoke or to the valve stem, and correlating the sensing device output to the known load. Later, the measured valve yoke strain is used to infer valve load. This method suffers from data uncertainty resulting from variables in the calibration method. Calibration of sensor output to stem thrust must be performed under field conditions which are often harsh. Also, the relationship between stem thrust and yoke strain can change due to changes in mechanical conditions, making frequent recalibration necessary. This device is attached to the exterior of the yoke in a somewhat fragile manner (by soldering, brazing, welding, epoxying or gluing).
Branam utilizes load sensors, in parallel loading with actuator mounting bolts, between the actuator and valve yoke. The Branam sensors carry only part of the load between the actuator and valve yoke, with the bolts carrying the remainder. Due to this mounting scheme the load sensors must be calibrated after the bolts are tightened to a desired, but variable level. This variable preloading of the load sensors results in a correction factor which must be computed for each valve operator assembly, and varies depending on the bolt material used, the size of the bolts, and the free length of the bolted connection. Also, the preload applied to the fasteners is liable to change due to small mechanical shifts in the thread contact areas during testing.
The prior art sensing devices previously discussed are designed primarily for operation on MOVs of the rising stem type, where the valve stem travels axially to raise or lower the valve plug or gate within the valve, opening or closing the valve respectively. Rising stem valves can be separated into two categories. The "rising non-rotating stem" design has a valve stem which is restrained from rotation and rises due to rotation of a threaded valve stem nut around a threaded section of the valve stem. In a "rising rotating" valve stem configuration, a threaded valve yoke nut is rigidly mounted in the center of the valve yoke flange, and the valve stem rotates due to the interaction of a splined stem nut, which is rigidly attached to one end of the valve stem, with a splined drive sleeve. As the threaded valve stem rotates against the threads on the yoke nut, it is forced to move axially, and the stem nut slides axially within the drive sleeve. The discussed, prior art devices appear to be of little value in measuring stem associated with MOV types in which the valve plug does not rise, but rotates ninety degrees to move between an open or closed position. This "quarterturn" or "rotating" valve experiences negligible thrust when operated and a torque-only sensor is needed to monitor and evaluate valve operating performance.